REPTILIA. 251 



adhesion is effected by appression of the dilated extremities of their 

 fingers and toes and by a viscosity ; see v. WITTICH Der Mechanis- 

 mus der HaftzeJien von Hyla arborea, MULLER'S Arcftiv, 1854, s. 

 170 183, PL viii. figs. 2, 3. The species are very numerous, but, 

 with the exception of a single one, foreign to our quarter of the 

 world; more than half of those now known are natives of the new 

 world. The skin is mostly smooth on the back, whilst on the abdo- 

 minal surface and along the inside of the legs it is beset with small 

 tubercles or warts placed close together. The colours are often 

 lively and are also variable. Hyla viridis has two kinds of pigment- 

 cells beneath the skin which are essentially different, and one of 

 them consists of two subdivisions. The one kind is irregularly 

 polyhedral and filled -with gold-yellow pigment-granules. They 

 never change their form. The cells of the other kind are starred or 

 polyhedral of variable form ; one class of these is black, the other 

 light brown. The difference of colour depends upon the mass of the 

 contained pigment -molecules, which singly are light brown. The 

 brown pigment-cells exhibit interference-colours that belong to the 

 third Newtonian system of rings. The black pigment-cells shew 

 here and there a shade of blue. The yellow pigment-cells are placed 

 the deepest below the cuticle. The brown pigment-cells in particu- 

 lar places have long anastomosing processes, and under different cir- 

 cumstances of excitement the pigment-granules are caused to pass 

 from the processes, which then contain only a fluid on which the in- 

 terference-colours depend. Thus the change of colour arises from the 

 variable quantity of the brown molecules in the more superficial 

 layer of cells, which on the one hand allows of a greater or less trans- 

 parency for the yellow cells below them to be seen, and on the other 

 a quantity of fluid, more or less unmixed with pigment-molecules, 

 for the production of the interference-colours. See HARLESS Ueber 

 die Chromatophoren des Frosches, Zeitschr.f. wissensch. Zool. v. 1854, 

 pp. 373 379. For the skin of Rana temporaria see A. HEXSCHE 

 Ueber die Drusen und glatten Muskeln in deraussern Haut von 

 Rana temporaria, Zeitschr.f. wissensch. Zool. vn. 1855, 273 282. 



On this genus see A. DUMERIL Mem. sur les Batraciens anoures 

 de la Famille des Hylceformes ou Rainettes, Ann. des sc. nat. 

 serie, xix. 1853, p. 135 179. 



* With membrane of tympanum covered, latent. 

 Microhyla TSCHUDI. 

 Theloderma TSCHUDI. 



